The present invention relates to a diode having a semiconductor substrate that is situated between two metallic electrodes and that is strongly doped in a first zone in order to form an ohmic transition to the first electrode, and is weakly doped in a second zone having the same conductivity type in order to form a rectifying transition to the second electrode.
Semiconductor diodes of this type, also known as Schottky diodes, are conventional. They are distinguished by a small voltage drop in the conducting direction and a short turn-off time, because, in contrast to pn diodes or pin diodes, no minority charge carriers need be discharged in order to stop a flow of current.
FIG. 4 illustrates a simple example embodiment of such a diode. Above a strongly doped zone 3 is arranegd a more weakly doped zone 1. A thin metal layer, made, for example, of aluminum, is applied to each of the two zones. The metal layer on the lower side of the substrate forms a first electrode 6, which is in ohmic contact with zone 3 of the semiconductor substrate arranged above it. The metal layer on the upper side of the semiconductor constitutes a second electrode 5 which forms, together with zone 1, a metal semiconductor contact having a diode characteristic. First electrode 6 represents the cathode, and second electrode 5 represents the anode of the diode.
If such a component is operated in the reverse direction, then, at a certain boundary voltage, a sharp increase in the reverse current occurs as a result of avalanche multiplication, analogously to a one-sided abrupt pn transition. However, the boundary voltages, at which such an increase in the current occurs, are mostly significantly smaller than would be expected proportionately to the selected doping of zone 1. The deviation is typically of a factor of 3. The reason for this is that a rise in field strength occurs at the edges of electrodes 5, 6. For this reason, the avalanche multiplication begins at the edge of the component. The result is that diodes having the configuration illustrated in FIG. 4 exhibit high reverse currents already below the breakdown voltage. In the case of an avalanche breakdown, high power losses occur at the diode edge, because the overall breakdown current is concentrated at this region. For this reason, diodes having the simple configuration illustrated in FIG. 4 are not suitable for use as elements for limiting voltage.
A conventional solution to this problem is the configuration illustrated in FIG. 5. This configuration is described, for example, in B. J. Baliga, Power Semiconductor Devices, PWS Publishing Company, Boston, U.S., 1995. An annular, circumferential p-doped layer 7 is additionally introduced into n-doped zone 1. In accordance with steps that are standard in planar technology, anode 5 is fashioned so that, on the one hand, it is contacted with n-doped second zone 1 and with p-doped layer 7, and that, on the other hand, the outer edge of anode 5 comes to rest on an oxide layer 8 on the surface of the semiconductor substrate. The circumferential p-doped layer 7 is called a guard ring. In this manner, a reduction in the edge field strength is achieved. The avalanche breakdown now no longer occurs at the edge, but rather is distributed in a uniform manner over the surface of second zone 1 inside guard ring 7. Because no local breakdowns occur at the edge at voltages below the desired breakdown boundary voltage, a Schottky diode having a guard ring can be used for voltage limitation.
The manufacture of such a diode is, however, associated with an increased expense. Thus, on the one hand, the manufacture of a flat, weakly doped zone such as zone 1 over a more strongly doped zone, such as zone 3, is expensive, because, in general, an epitaxial method must be used for this purpose. Subsequently, guard ring 7 must be structured and put in place, and oxide layer 8 must be structured, in order finally to enable anode 5 to be deposited thereupon in the desired form.
In accordance with the present intention, a diode of the type indicated above is created that is suitable for use as a voltage limiter and may be manufactured easily and economically. These advantages are achieved due to the fact that, in the diode according to the present invention, the first and second zone are separated by a third zone of the semiconductor substrate, this third zone, having the same conductivity type as the two others, being doped more weakly than the second zone.
Through suitable choice of the dimensions and doping concentrations of the individual zones, it may be ensured that the breakdown voltage at the transition from the second electrode to the third zone is greater than to the more strongly doped second zone. As a result, when the breakdown voltage of this second zone is achieved, the edge field strength at an edge of the second electrode touching the third zone is smaller than in its region touching the second zone, so that an avalanche breakdown occurs only in the second zone.
The conventional guard ring, and the process steps required for its manufacture, may therefore be omitted. Since the diode requires only zones having the same conductivity type, a single doping agent is sufficient.
The dimensions and the dopings of the zones may be selected such that the (calculated) breakdown voltage in a contact region between the second electrode and the third zone is at least three times as great as that between the second electrode and the second zone.
According to a first example embodiment of the present invention, the second zone is raised over the surface of the third zone, and the second electrode covers the second zone in a hat shape, and has a surrounding rim that touches the second zone. Such a diode may be produced, for example, using a manufacturing process where, first, the second zone is produced on the overall surface of the third zone of the semiconductor substrate and is subsequently eroded locally, in order to expose the surface of the third zone locally.
This local erosion may include a sawing using a circular saw, or also a masking and etching method.
According to a second example embodiment of the present invention, the surface of the diode may also be planar, and the second zone may be embedded into the third zone in the manner of islands, and the second electrode is flat and touches the third zone in an edge region. Such a diode may be produced, for example, through the island-by-island application of a doping agent onto the surface of the semiconductor substrate, doped with the concentration of the third zone, and diffusing in of the doping agent.
In order to improve the contact between the electrodes and the semiconductor substrate, at least one of the electrodes may be applied to an oxide-free surface of the semiconductor substrate. In order to remove the oxide that is naturally present on a semiconductor crystal, a treatment of the surface through sputtering, through heating in an ultrahigh vacuum, or through suitable etching is possible. A sputter treatment, for example using argon ions, is in particular simple and useful if the electrodes are subsequently also to be produced through the sputtering of metal onto the semiconductor substrate.
Additional features and advantages of the invention are derived from the following description of example embodiments, with reference to the Figures.